Process for liquefying a natural gas stream containing at least one freezable component

ABSTRACT

This invention relates to a process for producing pressurized liquid rich in methane from a multi-component feed stream containing methane and a freezable component having a relative volatility less than that of methane. The multi-component feed stream is introduced into a separation system having a freezing section operating at a pressure above about 1,380 kPa (200 psia) and under solids forming conditions for the freezable component and a distillation section positioned below the freezing section. The separation system produces a vapor stream rich in methane and a liquid stream rich in the freezable component. At least a portion of the vapor stream is cooled to produce a liquefied stream rich in methane having a temperature above about -112° C. (-170° F.) and a pressure sufficient for the liquid product to be at or below its bubble point. A first portion of the liquefied stream is returned to the separation system to provide refrigeration to the separation system. A second portion of the liquefied stream is withdrawn as a pressurized liquefied product stream rich in methane having a temperature above about -112° C. (-170° F.).

This application claims the benefit of (i) U.S. Provisional ApplicationNo. 60/051,460, filed July 1, 1997; and (ii) U.S. ProvisionalApplication No. 60/087,677, filed Jun. 2, 1998.

FIELD OF THE INVENTION

This invention relates to a natural gas liquefaction process, and moreparticularly relates to a process to produce pressurized liquid naturalgas (PLNG) from a natural gas stream containing at least one freezablecomponent.

BACKGROUND OF THE INVENTION

Because of its clean burning qualities and convenience, natural gas hasbecome widely used in recent years. Many sources of natural gas arelocated in remote areas, great distances from any commercial markets forthe gas. Sometimes a pipeline is available for transporting producednatural gas to a commercial market. When pipeline transportation is notfeasible, produced natural gas is often processed into liquefied naturalgas (which is called "LNG") for transport to market.

One of the distinguishing features of a LNG plant is the large capitalinvestment required for the plant. The equipment used to liquefy naturalgas is generally quite expensive. The liquefaction plant is made up ofseveral basic systems, including gas treatment to remove impurities,liquefaction, refrigeration, power facilities, and storage and shiploading facilities. While the cost of LNG plant can vary widelydepending upon plant location, a typical conventional LNG project cancost from U.S. $5 billion to U.S. $10 billion, including fielddevelopment costs. The plant's refrigeration systems can account for upto 30 percent of the cost.

LNG refrigeration systems are expensive because so much refrigeration isneeded to liquefy natural gas. A typical natural gas stream enters a LNGplant at pressures from about 4,830 kPa (700 psia) to about 7,600 kPa(1,100 psia) and temperatures from about 20° C. to about 40° C. Naturalgas, which is predominantly methane, cannot be liquefied by simplyincreasing the pressure, as is the case with heavier hydrocarbons usedfor energy purposes. The critical temperature of methane is -82.5° C.This means that methane can only be liquefied below that temperatureregardless of the pressure applied. Since natural gas is a mixture ofgases, it liquefies over a range of temperatures. The criticaltemperature of natural gas is between about -85° C. and -62° C.Typically, natural gas compositions at atmospheric pressure will liquefyin the temperature range between about -165° C. and -155° C. Sincerefrigeration equipment represents such a significant part of the LNGfacility cost, considerable effort has been made to reduce refrigerationcosts.

Many systems exist in the prior art for the liquefaction of natural gasby sequentially passing the gas at an elevated pressure through aplurality of cooling stages whereupon the gas is cooled to successivelylower temperatures until the gas liquefies. Conventional liquefactioncools the gas to a temperature of about -160° C. at or near atmosphericpressure. Cooling is generally accomplished by heat exchange with one ormore refrigerants such as propane, propylene, ethane, ethylene, andmethane. Although many refrigeration cycles have been used to liquefynatural gas, the three types most commonly used in LNG plants today are:(1) "cascade cycle" which uses multiple single component refrigerants inheat exchangers arranged progressively to reduce the temperature of thegas to a liquefaction temperature, (2) "expander cycle" which expandsgas from a high pressure to a low pressure with a correspondingreduction in temperature, and (3) "multi-component refrigeration cycle"which uses a multi-component refrigerant in specially designedexchangers. Most natural gas liquefaction cycles use variations orcombinations of these three basic types.

In conventional LNG plants water, carbon dioxide, sulfur-containingcompounds, such as hydrogen sulfide and other acid gases, n-pentane andheavier hydrocarbons, including benzene, must be substantially removedfrom the natural gas processing, down to parts-per-million (ppm) levels.Some of these compounds will freeze, causing plugging problems in theprocess equipment. Other compounds, such as those containing sulfur, aretypically removed to meet sales specifications. In a conventional LNGplant, gas treating equipment is required to remove the carbon dioxideand acid gases. The gas treating equipment typically uses a chemicaland/or physical solvent regenerative process and requires a significantcapital investment. Also, the operating expenses are high. Dry beddehydrators, such as molecular sieves, are required to remove the watervapor. A scrub column and fractionation equipment are used to remove thehydrocarbons that tend to cause plugging problems. Mercury is alsoremoved in a conventional LNG plant since it can cause failures inequipment constructed of aluminum. In addition, a large portion of thenitrogen that may be present in natural gas is removed after processingsince nitrogen will not remain in the liquid phase during transport ofconventional LNG and having nitrogen vapors in LNG containers at thepoint of delivery is undesirable.

There is a continuing need in the industry for an improved process forliquefying natural gas that contains CO₂ in concentrations that wouldfreeze during the liquefaction process and at the same time having powerrequirements that are economic.

SUMMARY

The invention relates generally to a process for producing pressurizedliquefied natural gas (PLNG) in which the natural gas feed streamcontains a freezable component. The freezable component, althoughtypically CO₂, H₂ S or another acid gas, can be any component that hasthe potential for forming solids in the separation system.

In the process of this invention, a multi-component feed streamcontaining methane and a freezable component having a relativevolatility less than that of methane is introduced into a separationsystem having a freezing section operating at a pressure above about1,380 kPa (200 psia) and under solids forming conditions for thefreezable component and a distillation section positioned below thefreezing section. The separation system, which contains a controlledfreezing zone ("CFZ"), produces a vapor stream rich in methane and aliquid stream rich in the freezable component. At least a portion of thevapor stream is cooled to produce a liquefied stream rich in methanehaving a temperature above about -112° C. (-170° F.) and a pressuresufficient for the liquid product to be at or below its bubble point. Afirst portion of the liquefied stream is withdrawn from the process as apressurized liquefied product stream (PLNG). A second portion of theliquefied stream is returned to the separation system to providerefrigeration duty to the separation system.

In one embodiment, a vapor stream is withdrawn from an upper region ofthe separation system and is compressed to a higher pressure and cooled.The cooled, compressed stream is then expanded by an expansion means toproduce a predominantly liquid stream. A first portion of the liquidstream is fed as a reflux stream to the separation system, therebyproviding open-loop refrigeration to the separation system, and a secondportion of the liquid stream is withdrawn as a product stream having atemperature above about -112° C. (-170° F.) and a pressure sufficientfor the liquid product to be at or below its bubble point.

In another embodiment, a vapor stream is withdrawn from an upper regionof the separation system and cooled by a closed-loop refrigerationsystem to liquefy the methane-rich vapor stream to produce a liquidhaving a temperature above about -112° C. (-170° F.) and a pressuresufficient for the liquid product to be at or below its bubble point.

The method of the present invention can be used both for the initialliquefaction of a natural gas at the source of supply for storage ortransportation, and to re-liquefy natural gas vapors given off duringstorage and ship loading. Accordingly, an object of this invention is toprovide an improved, integrated liquefaction and CO₂ removal system forthe liquefaction or reliquefaction of natural gas with high CO₂concentrations (greater than about 5%). Another object of this inventionis to provide an improved liquefaction system wherein substantially lesscompression power is required than in prior art systems. A still furtherobject of the invention is to provide a more efficient liquefactionprocess by keeping the process temperature for the entire process aboveabout -112° C., thereby enabling the process equipment to be made ofless expensive materials than would be required in a conventional LNGprocess that have at least part of the process operating at temperaturesdown to about -160° C. The very low temperature refrigeration ofconventional LNG process is very expensive compared to the relativelymild refrigeration needed in the production of PLNG in accordance withthe practice of this invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention and its advantages will be better understood byreferring to the following detailed description and the attached Figureswhich are schematic flow diagrams of representative embodiments of thisinvention.

FIG. 1 is a schematic representation of a cryogenic, CFZ processgenerally illustrating a closed-loop refrigeration cycle for producingpressurized liquefied natural gas in accordance with the process of thisinvention.

FIG. 2 is a schematic representation of a cryogenic, CFZ processgenerally illustrating an open-loop refrigeration cycle for producingpressurized liquefied natural gas in accordance with the process of thisinvention.

FIG. 3 is a schematic representation of still another embodiment of thepresent invention in which carbon dioxide and methane are distillativelyseparated in a distillation column having a CFZ in which one overheadproduct stream is pressurized liquefied natural gas and another overheadproduct stream is product sales gas.

The flow diagrams illustrated in the Figures present various embodimentsof practicing the process of this invention. The Figures are notintended to exclude from the scope of the invention other embodimentsthat are the result of normal and expected modifications of thesespecific embodiments. Various required subsystems such as pumps, valves,flow stream mixers, control systems, and sensors have been deleted fromthe Figures for the purposes of simplicity and clarity of presentation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process of this invention distillatively separates in a separationsystem a multi-component feed stream containing methane and at least onefreezable component having a relative volatility less than that ofmethane, wherein the separation system contains a controlled freezingzone ("CFZ"). The separation system produces an overhead vapor streamenriched with methane and a bottoms product enriched with the freezablecomponent. At least part of the overhead vapor stream is then liquefiedto produce liquefied natural gas product having a temperature aboveabout -112° C. (-170° F.) and a pressure sufficient for the liquidproduct to be at or below its bubble point. This product is sometimesreferred to herein as pressurized liquid natural gas ("PLNG"). Anotherportion of such liquefied overhead stream is returned to the separationsystem as a reflux stream.

The term "bubble point" is the temperature and pressure at which aliquid begins to convert to gas. For example, if a certain volume ofPLNG is held at constant pressure, but its temperature is increased, thetemperature at which bubbles of gas begin to form in the PLNG is thebubble point. Similarly, if a certain volume of PLNG is held at constanttemperature but the pressure is reduced, the pressure at which gasbegins to form defines the bubble point. At the bubble point, PLNG issaturated liquid. It is preferred that the PLNG is not just condensed toits bubble point, but further cooled to subcool the liquid. Subcoolingthe PLNG reduces the amount of boil-off vapors during its storage,transportation and handling.

Prior to this invention, it was well understood by those skilled in theart that CFZ could remove unwanted CO₂. It was not appreciated that theCFZ process could be integrated with a liquefaction process to producePLNG.

The process of the present invention is more economic to use since theprocess requires less power for liquefying the natural gas thanprocesses used in the past and the equipment used in the process of thisinvention can be made of less expensive materials. By contrast, priorart processes that produce LNG at atmospheric pressures havingtemperatures as low as -160° C. require process equipment made ofexpensive materials for safe operation.

In the practice of this invention, the energy needed for liquefyingnatural gas containing significant concentrations of a freezablecomponent such as CO₂ is greatly reduced over energy requirements of aconventional process for producing LNG from such natural gas. Thereduction in necessary refrigeration energy required for the process ofthe present invention results in a large reduction in capital costs,proportionately lower operating expenses, and increased efficiency andreliability, thus greatly enhancing the economics of producing liquefiednatural gas.

At the operating pressures and temperatures of the present invention,about 31/2 weight percent nickel can be used in piping and facilities inthe coldest operating areas of the liquefaction process, whereas themore expensive 9 weight percent nickel or aluminum is generally requiredfor the same equipment in a conventional LNG process. This providesanother significant cost reduction for the process of this inventioncompared to prior art LNG processes.

The first consideration in cryogenic processing of natural gas iscontamination. The raw natural gas feed stock suitable for the processof this invention may comprise natural gas obtained from a crude oilwell (associated gas) or from a gas well (non-associated gas). The rawnatural gas often contains water, carbon dioxide, hydrogen sulfide,nitrogen, butane, hydrocarbons of six or more carbon atoms, dirt, ironsulfide, wax, and crude oil. The solubilities of these contaminants varywith temperature, pressure, and composition. At cryogenic temperatures,CO₂, water, and other contaminants can form solids, which can plug flowpassages in cryogenic heat exchangers. These potential difficulties canbe avoided by removing such contaminants if conditions within their purecomponent, solid phase temperature-pressure phase boundaries areanticipated. In the following description of the invention, it isassumed that the natural gas stream contains CO₂. If the natural gasstream contains heavy hydrocarbons which could freeze out duringliquefaction, these heavy hydrocarbons will be removed with the CO₂.

One advantage of the present invention is that the warmer operatingtemperatures enables the natural gas to have higher concentration levelsof freezable components than would be possible in a conventional LNGprocess. For example, in a conventional LNG plant that produces LNG at-160° C., the CO₂ must be below about 50 ppm to avoid freezing problems.In contrast, by keeping the process temperatures above about -112° C.,the natural gas can contain CO₂ at levels as high as about 1.4 mole %CO₂ at temperatures of -112° C. and about 4.2% at -95° C. withoutcausing freezing problems in the liquefaction process of this invention.

Additionally, moderate amounts of nitrogen in the natural gas need notbe removed in the process of this invention because nitrogen will remainin the liquid phase with the liquefied hydrocarbons at the operatingpressures and temperatures of the present invention. The ability toreduce, or in some cases omit, the equipment required for gas treatingand nitrogen rejection provides significant technical and economicadvantages. These and other advantages of the invention will be betterunderstood by referring to the liquefaction process illustrated in theFigures.

Referring to FIG. 1, a natural gas feed stream 10 enters the system at apressure above about 3,100 kPa (450 psia) and more preferably aboveabout 4,800 kPa (700 psia) and temperatures preferably between about 0°C. and 40° C.; however, different pressures and temperatures can beused, if desired, and the system can be modified accordingly. If the gasstream 10 is below about 1,380 kPa (200 psia), it can be pressurized bya suitable compression means (not shown), which may comprise one or morecompressors. In this description of the process of this invention, it isassumed that the natural gas stream 10 has been suitably treated toremove water using conventional and well known processes (not shown inFIG. 1) to produce a "dry" natural gas stream.

Feed stream 10 is passed through cooler 30. The cooler 30 may compriseone or more conventional heat exchangers that cool the natural gasstream to cryogenic temperatures, preferably down to about -50° C. to-70° C. and more preferably to temperatures just above thesolidification temperature of CO₂. The cooler 30 may comprise one ormore heat exchange systems cooled by conventional refrigeration systems,one or more expansion means such as Joule-Thomson valves orturboexpanders, one or more heat exchangers which use liquid from thelower section of the fractionation column 31 as coolant, one or moreheat exchangers that use the bottoms product stream of column 31 ascoolant, or any other suitable source of cooling. The preferred coolingsystem will depend on the availability of refrigeration cooling, spacelimitation, if any, and environmental and safety considerations. Thoseskilled in the art can select a suitable cooling system taking intoaccount the operating circumstance of the liquefaction process.

The cooled stream 11 exiting the feed cooler 30 is conveyed into afractionation column 31 having a controlled freeze zone ("CFZ"), whichis a special section to handle solidification and melting of CO₂. TheCFZ section, which handles solidification and melting of CO₂, does notcontain packing or trays like conventional distillation columns, insteadit contains one or more spray nozzles and a melting tray. Solid CO₂forms in the vapor space in the distillation column and falls into theliquid on the melting tray. Substantially all of the solids that formare confined to the CFZ section. The distillation column 31 has aconventional distillation section below the CFZ section and preferablyanother distillation section above the CFZ section. Design and operationof a fractionation column 31 are known to those skilled in the art.Examples of CFZ designs are illustrated in U.S. Pat. Nos. 4,533,372;4,923,493; 5,062,270; 5,120,338; and 5,265,428.

A CO₂ -rich stream 12 exits the bottom of column 31. The liquid bottomproduct is heated in a reboiler 35 and a portion is returned to thelower section of column 31 as reboiled vapor. The remaining portion(stream 13) leaves the process as CO₂ -rich product. A methane-richstream 14 exits the top of column 31 and passes through a heat exchanger32 which is cooled by stream 17 that is connected to a conventionalclosed-loop refrigeration system 33. A single, multi-component, orcascade refrigeration system may be used. A cascade refrigeration systemwould comprise at least two closed-loop refrigeration cycles. Theclosed-loop refrigeration system may use as refrigerants methane,ethane, propane, butane, pentane, carbon dioxide, hydrogen sulfide, andnitrogen. Preferably, the closed-loop refrigeration system uses propaneas the predominant refrigerant. Although FIG. 1 shows only one heatexchanger 32, in the practice of this invention multiple heat exchangersmay be used to cool the vapor stream 14 in multiple stages. Heatexchanger 32 preferably condenses substantially all of vapor stream 14to a liquid. Stream 19 exiting the heat exchanger has a temperatureabove about -112° C. and a pressure sufficient for the liquid product tobe at or below its bubble point. A first portion of the liquid stream 19is passed as stream 20 to a suitable storage means 34 such as astationary storage tank or a carrier such as a PLNG ship, truck, orrailcar for containing the PLNG at a temperature above about -112° C.and a pressure sufficient for the liquid product to be at or below itsbubble point. A second portion of the liquid stream 19 is returned asstream 21 to the separation column 31 to provide refrigeration to theseparation column 31. The relative proportions of streams 20 and 21 willdepend on the composition of the feed gas 10, operating conditions ofthe separation column 31, and desired product specifications.

In the storage, transportation, and handling of liquefied natural gas,there can be a considerable amount of "boil-off," the vapors resultingfrom evaporation of liquefied natural gas. The process of this inventioncan optionally re-liquefy boil-off vapor that is rich in methane.Referring to FIG. 1, boil-off vapor stream 16 may optionally beintroduced to vapor stream 14 prior to cooling by heat exchanger 32. Theboil-off vapor stream 16 should be at or near the pressure of the vaporstream 14 to which the boil-off vapor is introduced. Depending on thepressure of the boil-off vapor, the boil-off vapor may need to bepressure adjusted by one or more compressors or expanders (not shown inthe Figures) to match the pressure at the point the boil-off vaporenters the liquefaction process.

A minor portion of the vapor stream 14 may optionally be removed fromthe process as fuel (stream 15) to supply a portion of the power neededto drive compressors and pumps in the liquefaction process. This fuelmay optionally be used as a refrigeration source to assist in coolingthe feed stream 10.

FIG. 2 illustrates in schematic form another embodiment of thisinvention in which open-loop refrigeration is used to providerefrigeration to the separation column 51 and to produce PLNG. Referringto FIG. 2, a multi-component gas stream 50 containing methane and carbondioxide that has been dehydrated and cooled by any suitable source ofcooling (not shown in FIG. 2) is fed into a CFZ column 51 which hasessentially the same design as separation column 31 of FIG. 1. Thisembodiment effectively manages the potential for the formation of solidsin the liquefaction process by feeding stream 64 directly into CFZcolumn 51.

The temperature of the gas fed into CFZ column 51 is preferably abovethe CO₂ solidification temperature. A methane-enriched vapor stream 52exits the overhead of CFZ column 51 and a carbon dioxide-enriched stream53 exits the bottom of CFZ column 51. The liquid bottom product isheated in a reboiler 65 and a portion is returned to the lower sectionof the CFZ column 51 as reboiled vapor. The remaining portion (stream54) leaves the process as CO₂ -rich liquid product.

A first portion of the overhead stream 52 is refluxed back to the CFZcolumn 51 as stream 64 to provide open-loop refrigeration to the CFZcolumn 51. A second portion of the overhead stream 52 is withdrawn(stream 63) as a PLNG product stream at a pressure that is at or nearthe operating pressure of the CFZ column 51 and at atemperature aboveabout -112° C. (-170° F.). A third portion of the overhead stream 52 mayoptionally be withdrawn (stream 59) for use as sales gas or furtherprocessed.

The principal components of open-loop refrigeration in this embodimentcomprise compressing by one or more compressors 57 the overhead stream52 exiting the top of the CFZ column 51, cooling the compressed gas byone or more coolers 58, passing at least part of the cooled gas (stream61) to one or more expansion means 62 to decrease the pressure of thegas stream and to cool it, and feeding a portion (stream 64) of thecooled, expanded stream to the CFZ column 51. Refluxing part of theoverhead stream 52 by this process provides open-loop refrigeration toCFZ column 51. Stream 60 is preferably cooled by heat exchanger 55 whichalso warms the overhead stream 52. The pressure of stream 64 ispreferably controlled by regulating the amount of compression producedby compressor 57 to ensure that the fluid pressures of streams 60, 61,and 64 are high enough to prevent formation of solids. Returning atleast part of the overhead vapor stream 52 to the upper portion ofcolumn 51 as liquid, condensed by open-loop refrigeration, also providesreflux to column 51.

CFZ column 51 has a conventional distillation section below the CFZsection and potentially another distillation section above the CFZsection. The CFZ section handles any formation and melting of CO₂solids. During start-up, all of stream 64 may be diverted directly tothe CFZ section. As stream 64 becomes leaner in the solids formers, moreof stream 64 can be fed to the distillation section of the column abovethe CFZ section.

FIG. 3 illustrates in schematic form another embodiment of thisinvention in which the process of this invention produces both PLNG andsales gas as product streams. In this embodiment, the overhead productstreams are 50% PLNG (stream 126) and 50% sales gas (stream 110).However, additional PLNG, up to 100%, can be produced by providingadditional cooling from either heat exchange with colder fluids oradditional pressure drop at the expander through the installation ofadditional compression and after-coolers. Likewise, less PLNG can beproduced by providing less cooling.

Referring to FIG. 3, it is assumed that natural gas feed stream 101contains over 5 mole % CO₂ and is virtually free of water to preventfreeze-ups and hydrate formation from occurring in the process. Afterdehydration, the feed stream is cooled, depressurized, and fed todistillation column 190 operating at a pressure in the range of fromabout 1,379 kPa (200 psia) to about 4,482 kPa (650 psia). Thedistillation column 190, which has a CFZ section similar to separationcolumn 31 of FIG. 1, separates the feed into a methane-enriched vaporoverhead product and a carbon dioxide-enriched liquid bottoms product.In the practice of this invention, distillation column 190 has at leasttwo, and preferably three, distinct sections: a distillation section193, a controlled freeze zone (CFZ) 192 above the distillation section193, and optionally an upper distillation section 191.

In this example, the tower feed is introduced into the upper part of thedistillation section 193 through stream 105 where it undergoes typicaldistillation. The distillation sections 191 and 193 contain trays and/orpacking and provide the necessary contact between liquids fallingdownward and vapors rising upward. The lighter vapors leave distillationsection 193 and enter the controlled freezing zone 192. Once in thecontrolled freezing zone 192, the vapors contact liquid (sprayedfreezing zone liquid reflux) emanating from nozzles or spray jetassemblies 194. The vapors then continue up through the upperdistillation section 191. For effective separation of CO₂ from thenatural gas stream in column 190, refrigeration is required to provideliquid traffic in the upper sections of the column 190. In the practiceof this embodiment, the refrigeration to the upper portion of column 190is supplied by open-loop refrigeration.

In the embodiment of FIG. 3, the incoming feed gas is divided into twostreams: stream 102 and stream 103. Stream 102 is cooled in one or moreheat exchangers. In this example, three heat exchangers 130, 131, 132are used to cool stream 102 and to serve as reboilers to provide heat tothe distillation section 193 of column 190. Stream 103 is cooled by oneor more heat exchangers that are in heat exchange with one of the bottomproduct streams of column 190. FIG. 3 shows two heat exchangers 133 and141 which warm bottoms products leaving the column 190. However, thenumber of heat exchangers for providing the feed stream cooling serviceswill depend on a number of factors including, but not limited to, inletgas flow rate, inlet gas composition, feed temperature, and heatexchange requirements. Optionally, although not shown in FIG. 3, feedstream 101 may be cooled by a process stream exiting the top of column190. As another option, the feed stream 101 may be cooled at leastpartially by conventional refrigeration systems, such as closed-loopsingle component or multi-component refrigeration systems.

Streams 102 and 103 are recombined and the combined stream is passedthrough an appropriate expansion means, such as Joule-Thomson valve 150,to approximately the operating pressure of the separation column 190.Alternatively, a turboexpander can be used in place of the Joule-Thomsonvalve 150. The flash expansion through valve 150 produces acold-expanded stream 105 which is directed to the upper part of thedistillation section 193 at a point where the temperature is preferablyhigh enough to avoid freezing of CO₂.

Overhead vapor stream 106 from the separation column 190 passes throughheat exchanger 145 which warms vapor stream 106. The warmed vapor stream(stream 107) is recompressed by single-stage compression or amulti-stage compressor train. In this example, stream 107 passessuccessively through two conventional compressors 160 and 161. Aftereach compression step, stream 107 is cooled by after-coolers 138 and139, preferably using ambient air or water as the cooling medium. Thecompression and cooling of stream 107 produces a gas which can be usedfor sale to a natural gas pipeline or further processing. Thecompression of vapor stream 107 will usually be to at least a pressurethat meets pipeline requirements.

A portion of stream 107 after passing through compressor 160 mayoptionally be withdrawn (stream 128) for use as fuel for the gasprocessing plant. Another portion of stream 107 after passing throughafter-cooler 139 is withdrawn (stream 110) as sales gas. The remainingpart of stream 107 is passed as stream 108 to heat exchangers 140, 136and 137. Stream 108 is cooled in heat exchangers 136 and 137 with coldfluids from stream 124 exiting the bottom of column 190. Stream 108 isthen cooled further in heat exchanger 145 by heat exchange with overheadvapor stream 106, resulting in warming of stream 106. Stream 108 is thenpressure expanded by an appropriate expansion device, such as expander158 to approximately the operating pressure of column 190. Stream 108then splits, one portion is passed as PLNG product (stream 126) at atemperature above about -112° C. and a pressure above about 1,380 kPa(200 psia) for storage or transportation. The other portion (stream 109)enters separation column 190. The discharge pressure of compressor 161is regulated to produce a pressure that is high enough so that thepressure drop across the expander 158 provides sufficient cooling toensure that streams 109 and 126 are predominantly liquid enriched inmethane. In order to produce additional PLNG (stream 126), additionalcompression can be installed after compressor 160 and before heatexchanger 136. To start up the process, stream 109 is preferably fedthrough stream 109A and sprayed directly into the CFZ section 192through spray nozzle 194. After process start up, stream 109 may be fed(stream 109B) to the upper section 191 of the separation column 190.

A CO₂ -enriched liquid product stream 115 exits the bottom of column190. Stream 115 is divided into two portions, stream 116 and stream 117.Stream 116 passes through an appropriate expansion device, such asJoule-Thomson valve 153, to a lower pressure. Stream 124 that exitsvalve 153 is then warmed in heat exchanger 136 and stream 124 passesthrough another Joule-Thomson valve 154 and still another heat exchanger137. The resulting stream 125 is then merged with vapor stream 120 fromseparator 181.

Stream 117 is expanded by an appropriate expansion device such asexpansion valve 151 and passed through heat exchanger 133 therebycooling feed stream 103. Stream 117 is then directed to separator 180, aconventional gas-liquid separation device. Vapor from separator 180(stream 118) passes through one or more compressors and high pressurepumps to boost the pressure. FIG. 3 shows a series of two compressors164 and 165 and pump 166 with conventional coolers 143 and 144. Productstream 122 leaving pump 166 in the series has a pressure and temperaturesuitable for injection into a subterranean formation. Liquid productsexiting separator 180 through stream 119 are passed through an expansiondevice such as expansion valve 152 and then passed through heatexchanger 141 which is in heat exchange relationship with feed stream103, thereby further cooling feed stream 103. Stream 119 is thendirected to separator 181, a conventional gas-liquid separator device.Vapors from separator 181 are passed (stream 120) to a compressor 163followed by a conventional after-cooler 142. Stream 120 is then mergedwith stream 118. Any condensate available in stream 121 may be recoveredby conventional flash or stabilization processes, and then may be sold,incinerated, or used for fuel.

Although the separation systems illustrated in FIGS. 1-3 have only onedistillation column (column 31 of FIG. 1, column 51 of FIG. 2, andcolumn 190 of FIG. 3), the separation systems of this invention cancomprise two or more distillation columns. For example, to reduce theheight of column 190 of FIG. 3, it may be desirable to split column 190into two or more columns (not shown in the figures). The first columncontains two sections, a distillation section and a controlled freezezone above the distillation section, and the second column contains onedistillation section, which performs the same function as section 191 inFIG. 3. A multi-component feed stream is fed to the first distillationcolumn. The liquid bottoms of the second column is fed to the freezingzone of the first column. The vapor overhead of the first column is fedto the lower region of the second column. The second column has the sameopen-loop refrigeration cycle as that shown in FIG. 3 for column 190. Avapor stream from the second distillation column is withdrawn, cooled,and a portion thereof refluxed to the upper region of the secondseparation column.

EXAMPLES

Simulated mass and energy balances were carried out to illustrate theembodiments shown in FIG. 1 and FIG. 3, and the results are shown inTables 1 and 2 below, respectively. For the data presented in Table 1,it was assumed that the overhead product stream was 100% PLNG (stream 20of FIG. 1) and the refrigeration system was a cascaded propane-ethylenesystem. For the data presented in Table 2, it was assumed that theoverhead product streams were 50% PLNG (stream 126 of FIG. 3) and 50%sales gas (stream 110 of FIG. 3).

The data were obtained using a commercially available process simulationprogram called HYSYS™ (available from Hyprotech Ltd. of Calgary,Canada); however, other commercially available process simulationprograms can be used to develop the data, including for example HYSIM™,PROII™, and ASPEN PLUS™, which are familiar to those of ordinary skillin the art. The data presented in the Tables are offered to provide abetter understanding of the embodiments shown in FIGS. 1 and 3, but theinvention is not to be construed as unnecessarily limited thereto. Thetemperatures and flow rates are not to be considered as limitations uponthe invention which can have many variations in temperatures and flowrates in view of the teachings herein.

An additional process simulation was done using the basic flow schemeshown in FIG. 1 (using the same feed stream composition and temperatureas used to obtain the data in Table 1) to produce conventional LNG atnear atmospheric pressure and a temperature of -161° C. (-258° F.). TheCFZ/conventional LNG process requires significantly more refrigerationthan the CFZIPLNG process depicted in FIG. 1. To obtain therefrigeration required to produce LNG at a temperature of -161° C., therefrigeration system must be expanded from a propane/ethylene cascadesystem to a propane/ethylene/methane cascade system. Additionally,stream 20 would need to be further cooled using the methane and theproduct pressure would need to be dropped using a liquid expander orJoule-Thomson valve to produce a LNG product at or near atmosphericpressure. Because of the lower temperatures, the CO₂ in the LNG must beremoved to about 50 ppm to avoid operational problems associated withfreezing of CO₂ in the process instead of 2% CO₂ as in the CFZ/PLNGprocess depicted in FIG. 1.

Table 3 shows a comparison of the refrigerant compression requirementsfor the conventional LNG process and the PLNG process described insimulation example of the foregoing paragraph. As shown in Table 3, thetotal required refrigerant compression power was 67% higher to produceconventional LNG than to produce PLNG in accordance with the practice ofthis invention.

A person skilled in the art, particularly one having the benefit of theteachings of this patent, will recognize many modifications andvariations to the specific processes disclosed above. For example, avariety of temperatures and pressures may be used in accordance with theinvention, depending on the overall design of the system and thecomposition of the feed gas. Also, the feed gas cooling train may besupplemented or reconfigured depending on the overall designrequirements to achieve optimum and efficient heat exchangerequirements. Additionally, certain process steps may be accomplished byadding devices that are interchangeable with the devices shown. Forexample, separating and cooling may be accomplished in a single device.As discussed above, the specifically disclosed embodiments and examplesshould not be used to limit or restrict the scope of the invention,which is to be determined by the claims below and their equivalents.

                                      TABLE 1    __________________________________________________________________________    Integrated CFZ/PLNG    Phase      Pressure                     Temperature                            Total Flow  Mole %    Steam        Vapor/Liquid               kPa                  psia                     ° C.                        ° F.                            kg-moles/hr                                  lb-moles/hr                                        CO.sub.2                                           CH.sub.4    __________________________________________________________________________    10  Vapor  6,764                  981                     18.3                        65.0                            49,805                                  109,800                                        71.1                                           26.6    11  Vapor/Liquid               3,103                  450                     -56.7                        -70.0                            49,805                                  109,806                                        71.1                                           26.6    12  Liquid 3,103                  450                     -7.7                        18.2                            55,656                                  122,700                                        95.9                                           1.4    13  Liquid 3,103                  450                     -4.9                        23.2                            36,424                                  80,300                                        96.5                                           0.5    14  Vapor  3,068                  445                     -92.0                        -133.6                            30,844                                  68,000                                        2.0                                           97.7    19  Liquid 3,068                  445                     -94.6                        -138.3                            30,844                                  68,000                                        2.0                                           97.7    20  Liquid 3,068                  445                     -94.6                        -138.3                            13,381                                  29,500                                        2.0                                           97.7    21  Liquid 3,068                  445                     -94.6                        -138.3                            17,463                                  38,500                                        2.0                                           97.7    __________________________________________________________________________

                                      TABLE 2    __________________________________________________________________________    Integrated CFZ/PLNG with open-loop refrigeration    Phase      Pressure                      Temperature                             Total Flow  Mole %    Steam        Vapor/Liquid               kPa psia                      ° C.                          ° F.                             kg-moles/hr                                   lb-moles/hr                                         CO.sub.2                                            N.sub.2                                               CH.sub.4                                                  H.sub.2 S                                                      C.sub.2 +    __________________________________________________________________________    101 Vapor  6,764                   981                      18.3                          65 49,850                                   109,900                                         71.1                                            0.4                                               26.6                                                  0.6 1.3    102 Vapor  6,764                   981                      18.3                          65 19,731                                   43,500                                         71.1                                            0.4                                               26.6                                                  0.6 1.3    103 Vapor  6,764                   981                      18.3                          65 30,119                                   66,400                                         71.1                                            0.4                                               26.6                                                  0.6 1.3    104 Vapor/Liquid               6,695                   971                      -7.8                          18 5,942 13,100                                         71.1                                            0.4                                               26.6                                                  0.6 1.3    105 Vapor/Liquid               2,758                   400                      -56.7                          -70                             49,850                                   109,900                                         71.1                                            0.4                                               26.6                                                  0.6 1.3    106 Vapor  2,758                   400                      -99.4                          -147                             31,116                                   68,600                                         0.1                                            1.5                                               98.4                                                  16 ppm                                                      0.0    107 Vapor  2,551                   370                      -30.6                          -23                             31,116                                   68,600                                         0.1                                            1.5                                               98.4                                                  16 ppm                                                      0.0    108 Vapor  16,823                   2,440                      51.7                          125                             23,723                                   52,300                                         0.1                                            1.5                                               98.4                                                  16 ppm                                                      0.0    109 Liquid 2,758                   400                      -101.7                          -151                             18,008                                   39,700                                         0.1                                            1.5                                               98.4                                                  16 ppm                                                      0.0    110 Vapor  16,823                   2,440                      51.7                          125                             5,715 12,600                                         0.1                                            1.5                                               98.4                                                  16 ppm                                                      0.0    115 Liquid 2,758                   400                      -11.1                          12 36,741                                   81,000                                         96.5                                            0.0                                               1.0                                                  0.7 1.8    116 Liquid 2,758                   400                      -11.1                          12 6,532 14,400                                         96.5                                            0.0                                               1.0                                                  0.7 1.8    117 Liquid 2,758                   400                      -11.1                          12 30,209                                   66,600                                         96.5                                            0.0                                               1.0                                                  0.7 1.8    118 Vapor  1,862                   270                      -21.1                          -6 21,727                                   47,900                                         96.8                                            0.0                                               1.3                                                  0.7 1.2    119 Liquid 1,862                   270                      -21.1                          -6 8,482 18,700                                         95.5                                            0.0                                               0.1                                                  0.9 3.5    120 Vapor  621 90 -23.3                          -10                             8,210 18,100                                         97.8                                            0.0                                               0.1                                                  0.9 1.2    121 Liquid 621 90 -23.3                          -10                             227   500   18.7                                            0.0                                               0.0                                                  0.6 80.7    122 Liquid 29,751                   4,315                      65.6                          150                             36,514                                   80,500                                         97.0                                            0.0                                               1.0                                                  0.7 1.3    123 Vapor  16,616                   2,410                      -28.3                          -19                             23,723                                   52,300                                         0.1                                            1.5                                               98.4                                                  16 ppm                                                      0.0    124 Vapor/Liquid               1,931                   280                      -22.2                          -8 6,532 14,400                                         96.5                                            0.0                                               1.0                                                  0.7 1.8    125 Vapor  621 90 -22.2                          -8 6,532 14,400                                         96.5                                            0.0                                               1.0                                                  0.7 1.8    126 Liquid 2,758                   400                      -101.7                          -151                             5,715 12,600                                         0.1                                            1.5                                               98.4                                                  16 ppm                                                      0.0    128 Vapor  6,895                   1,000                      56.1                          133                             l,633 3,600 0.1                                            1.5                                               98.4                                                  16 ppm                                                      0.0    __________________________________________________________________________

                                      TABLE 3    __________________________________________________________________________    Comparison of CFZ/Conventional LNG to CFZ/PLNG Refrigerant Compression    Power Requirements                     POWER, horsepower                                      POWER, kW                     CFZ/             CFZ/                     Conventional                           CFZ/PLNG                                 Difference                                      Conventional                                            CFZ/PLNG                                                  Difference    __________________________________________________________________________    Compressors    Propane Refrigerant Compressors                     162,220                           115,960                                 46,250                                      120,962                                            86,473                                                  34,489    Ethylene Refrigerant Compressors                     86,090                           41,490                                 44,600                                      64,198                                            30,940                                                  33,259    Methane Refrigerant Compressors                     14,031                           0     14,031                                      10,463                                            0     10,463    Total Installed Refrigerant Compression                     262,331                           157,450                                 104,881                                      195,623                                            117,412                                                  78,221    % of CFZ/PLNG    Total Installed  167%  100%  67%  167%  100%  67%    __________________________________________________________________________

What is claimed is:
 1. A process for producing pressurized liquid richin methane from a multi-component feed stream containing methane and afreezable component having a relative volatility less than that ofmethane, comprising:(a) introducing the multi-component feed stream intoa separation system having a freezing section operating at a pressureabove about 1,380 kPa (200 psia) and under solids forming conditions forthe freezable component and a distillation section positioned below thefreezing section, said separation system producing a vapor stream richin methane and a liquid stream rich in the freezable component; (b)cooling at least a portion of said vapor stream to produce a liquefiedstream rich in methane having a temperature above about -112° C. (-170°F.) and a pressure sufficient for the liquid product to be at or belowits bubble point; (c) withdrawing a first portion of the liquefiedstream of step (b) as a liquefied product stream rich in methane; and(d) introducing a second portion of the liquefied stream of step (b) tosaid separation system to provide refrigeration to said separationsystem.
 2. The process of claim 1 further comprising introducing theliquefied product stream to a storage means for storage at a temperatureabove -112° C. (-170° F.).
 3. The process of claim 1 wherein the coolingstep (b) further comprises the steps of compressing said vapor stream toa high pressure stream, cooling at least a portion of said compressedstream in a heat exchanger, and expanding the cooled, compressed streamto a lower pressure whereby the compressed stream is further cooled toproduce a liquefied stream rich in methane having a temperature aboveabout -112° C. (-170° F.) and a pressure sufficient for the liquidproduct to be at or below its bubble point.
 4. The process of claim 3wherein the cooling of the compressed stream in the heat exchanger is byindirect heat exchange with the vapor stream of step (a).
 5. The processof claim 3 further comprises cooling the liquid stream produced by saidseparation system by pressure expansion and using the expanded, cooledliquid stream to cool by indirect heat exchange the compressed stream.6. The process of claim 3 further comprises regulating the pressure ofthe compressed stream and the pressure of the expanded stream to preventformation of solids in the second portion of the liquefied streamintroduced to the separation system.
 7. The process of claim 1 whereinsaid separation system of step (a) comprises a first distillation columnand a second distillation column, said first distillation columncomprising a distillation section and a freezing zone above thedistillation section, said second distillation column comprising adistillation section, further comprising the steps of introducing saidmulti-component feed stream of step (a) into said first distillationcolumn, feeding a vapor overhead stream from said freezing zone to alower region of the second distillation column, withdrawing a vaporstream from the second distillation column and cooling said vapor streamin accordance with step (b), feeding the second portion of the liquefiedstream of step (d) to the upper region of said second separation column,withdrawing a liquid bottom stream from said second distillation column,and feeding the liquid bottom stream to said freezing zone of said firstdistillation column.
 8. The process of claim 1 in which the separationsystem comprises a first distillation section, a second distillationsection below the first distillation section, and a freezing zonebetween the first and the second distillation sections, wherein thesecond portion of the liquefied stream of step (d) is introduced to thefirst distillation section.
 9. The process of claim 1 wherein thecooling of said vapor stream in step (b) is effected in a heat exchangercooled by a closed-loop refrigeration system.
 10. The process of claim 9wherein the closed-loop refrigeration system has propane as thepredominant refrigerant.
 11. The process of claim 9 wherein theclosed-loop refrigeration system has a refrigerant comprising methane,ethane, propane, butane, pentane, carbon dioxide, hydrogen sulfide, andnitrogen.
 12. The process of claim 1 further comprises, prior to step(b), passing to said process boil-off gas resulting from evaporation ofliquefied gas rich in methane.
 13. The process of claim 1 wherein theliquefaction of the gas stream is performed using two closed-looprefrigeration cycles in cascade arrangement.
 14. The process of claim 1wherein the multi-component gas stream of step (b) has a pressure above3,100 kPa (450 psia).
 15. The process of claim 1 wherein the freezablecomponent is carbon dioxide.
 16. The process of claim 1 wherein thecooling step (b) further comprises the steps of compressing said vaporstream to a compressed stream, cooling at least a portion of saidcompressed stream in a heat exchanger, withdrawing a first portion ofthe cooled compressed stream as a product gas stream, and expanding asecond portion of the cooled compressed stream to a lower pressurewhereby the compressed stream is further cooled to produce a liquefiedstream rich in methane having a temperature above about -112° C. (-170°F.) and a pressure sufficient for the liquid product to be at or belowits bubble point.
 17. A process for separating a multi-component feedstream comprising at least methane and at least one freezable componenthaving a relative volatility less than that of methane to produce aliquid product enriched in methane, comprising:(a) introducing themulti-component feed stream into a separation system, said separationsystem operating under solids forming conditions for said freezablecomponent; (b) withdrawing a vapor stream from an upper region of saidseparation system; (c) compressing said vapor stream to a higherpressure stream; (d) cooling at least a portion of said compressedstream using the cooling available in vapor stream of step (b); (e)expanding said cooled compressed stream to further cool said compressedstream, said expanded stream being predominantly liquid; (f) feeding atleast a portion of said expanded stream to an upper region of theseparation system to provide refrigeration to said separation system;and (g) recovering from the expanded stream a liquid product streamenriched in methane.
 18. The process of claim 17 further comprisesrecovering a portion of said compressed vapor stream of step (c) andcooling the remaining portion of said vapor stream in accordance withstep (d).
 19. The process of claim 17 wherein said vapor stream of step(b) is warmed prior to compression in step (c).
 20. The process of claim17 in which the separation system comprises a first distillationsection, a second distillation section below the first distillationsection, and a freezing zone between the first and second distillationsections, wherein the expanded liquid stream is introduced into thefirst distillation section.
 21. The process of claim 20 wherein saidmulti-component feed stream is introduced below the first distillationsection.
 22. The process of claim 17 further comprising removing liquidfrom the separation system, cooling said liquid by a pressure expansionmeans, and at least partially vaporizing said liquid by heat exchangewith the compressed stream of step (c).
 23. The process of claim 17further comprising removing liquid from the separation system enrichedwith said freezable component, cooling said freezable component-enrichedliquid by a pressure expansion means, and cooling the multi-componentfeed stream before it enters the separation system by heat exchange withsaid expanded, freezable component-enriched liquid.
 24. The process ofclaim 17 further comprising cooling the multi-component stream by anexpansion means before it enter the separation system.
 25. The processof claim 17 wherein the pressure of the higher pressure stream of step(c) and the pressure of the expanded stream (e) are controlled toprevent solids formation in the stream fed to the separation system instep (f).
 26. The process of claim 17 wherein the recovered liquidproduct stream of step (g) has a pressure above about 1,380 kPa (200psia).
 27. A process for producing liquefied natural gas at a pressureabove about 1,380 kPa (200 psia) from a multi-component feed streamcontaining methane and a freezable component having a relativevolatility less than that of methane, comprising:(a) introducing themulti-component feed stream into a separation system, said separationsystem operating under solids forming conditions for said freezablecomponent; (b) withdrawing a vapor stream from an upper region of saidseparation system; (c) compressing said vapor stream to a higherpressure stream; (d) cooling at least a portion of said compressedstream using the cooling available in vapor stream of step (b); (e)expanding said cooled compressed stream to further cool said compressedstream, said expanded stream being predominantly liquid at a pressureabove about 1,380 kPa (200 psia); (f) feeding at least a portion of saidexpanded stream to an upper portion of the separation system to providerefrigeration to said separation system; and (g) recovering from theexpanded stream a liquid product stream enriched in methane at apressure above about 1,380 kPa (200 psia).
 28. A process for liquefyinga multi-component stream comprising methane and at least one freezablecomponent to produce a methane-rich liquid having a temperature aboveabout -112° C. and a pressure sufficient for the liquid to be at orbelow its bubble point, comprising the steps of:(a) introducing themulti-component feed stream having a pressure above about 1,380 kPa (200psia) into a separation system operating under solids forming conditionsfor said freezable component to provide a methane-rich vapor stream anda liquid stream rich in said component that solidified in the separationsystem; (b) liquefying the vapor stream by a closed loop refrigerationsystem to produce a methane-rich liquid having a temperature above about-112° C. and a pressure sufficient for the liquid to be at or below itsbubble point; and (c) introducing said methane-rich liquid to a storagevessel for storage at a temperature above -112° C.
 29. The process ofclaim 28 wherein the liquefaction of the feed stream is performed with aclosed-loop refrigeration system.
 30. The process of claim 28 whereinprior to liquefaction of the feed stream further comprises combiningwith the vapor stream from the separation system a boil-off gasresulting from evaporation of liquefied natural gas.